1. Field
The present invention relates to the growth of monocrystalline germanium (Ge) crystals, as well as systems, methods and substrates related thereto.
2. Description of Related Information
Electronic and opto-electronic device manufacturers routinely require large and electronically uniform single semiconductor crystals which, when sliced and polished, provide substrates for microelectronic device production. The growth of a semiconductor crystal involves heating polycrystalline raw material to its melting point (typically in excess of 1,200° C.) to create a polycrystalline raw material melt, bringing the melt into contact with a high quality seed crystal of the same material, and allowing the melt to crystallize on its contact surface with the seed crystal. A number of different processes for accomplishing this are known from the literature. These include the Czochralski process (Cz) and its variant, the Liquid Encapsulated Czochralski process (LEC), the Horizontal Bridgman and Bridgman-Stockbarger processes (HB) and their vertical variants (VB), and the gradient freeze process (GF) and its variant, the vertical gradient freeze processes (VGF). See for example “Bulk Crystal Growth of Electronic, Optical and Optoelectronic Materials”, P. Clapper, Ed., John Wiley and Sons Ltd, Chichester, England, 2005 for general discussions of these techniques and their application to the growth of various materials.
With regard to known processes, 150 mm (6 inch) diameter, low dislocation germanium single crystals have been produced commercially by using the Czochralski technique. Larger diameter wafer have been discussed, though not demonstrated (Vanhellemont and Simoen, J. Electrochemical Society, 154 (7) H572-H583 (2007). Further, 100 mm (4 inch) diameter germanium single crystals have been grown by the VGF and VB techniques as indicated in the literature (Ch. Frank-Rotsch, et al., J. Crystal Growth (2008), doi:10.1016, J. Crys Growth 2007.12.020).
As shown by many studies reported in the literature, compared with the Cz/LEC techniques, the VB/VGF growth techniques generally utilize lower thermal gradients and lower growth rates and, thus, produce single crystals with much lower dislocation densities (see A. S. Jordan et al., J. Cryst. Growth 128 (1993) 444-450, 2), M. Jurisch et al., J. Cryst. Growth 275 (2005) 283-291, and S. Kawarabayashi, 6th Intl. Conf. on InP and Related Materials (1994), 227-230). Thus, in certain applications, the VB/VGF processes may be preferred for the growth of large diameter, low dislocation density (or dislocation-free) germanium.
In commercial single crystal growth operations, the objectives are to grow an ingot at the lowest possible cost, and cut wafers from an ingot at a high yield, namely cut as many wafers as possible from an ingot. Therefore, if one wishes to grow the longest possible ingot, with all the other constraints imposed by the process, a large crucible size is often desired. Usually, the charge to be loaded into the crucible is generally made up of polycrystalline chunks of different shapes, and big voids between raw materials result with no material in them; thus, the loading co-efficient is low. Hence, when the charge is melted, the crucible is only partially filled with the melt. Considering the desired melt volume and the structure of the available crucible, supplementing the melt with additional material is an important step in the whole growing process, and is also a complicated step. Aspects, here, are especially relevant for certain materials, such as germanium, which is subject to special process restrictions owing to its lower thermal conductivity (0.58 W cm−1° C.−1) and higher density (5.32 g cm−3) as compared to Si (1.358 W cm−1° C.−1 and 2.3332 g cm−3, for thermal conductivity and density, respectively).
Melt replenishment in crystal growth is known in several rudimentary forms. In silicon growth systems, for example, wherein means for adding lump polycrystalline raw material to a Si melt exist for the growth of monocrystalline Si, as well as in systems wherein raw material is loaded into a crucible for Cz growth of single crystals. Techniques like these are possible because the Cz (or LEC) system is open and access to the crucible is relatively easy. However, with the VGF and VB techniques wherein the crucible is contained in a sealed ampoule, such processes cannot be used. Moreover, special requirements associated with growing particular, doped germanium single crystal may obviate the ability to use such processes. For instance, if As, which has a high vapor pressure and is toxic, is used as a dopant, use of such processes involving doping germanium single crystal with As may be restricted.